1. Field of the Invention
This invention relates to a fabrication process of an electro-luminescent device, and more particularly, to a fabrication process of a full-color organic electro-luminescent (OEL) device.
2. Description of Related Art
The organic electro-luminescent technique, which basically applies a high voltage on particular organic compounds, was originally introduced in 1963. Since the intensity of luminescence was far below the practical level, research on developing more practicable methods and materials has been done since then.
In 1987, Kodak-USA first successfully developed an electro-luminescent device by applying low voltage on a small molecule (Applied Physics letter, Volume 51, pp. 914, 1987). In 1990, the Cambridge University further developed an electro-luminescent device by using polymer as the light-emitting layer (Nature, Volume 347, pp. 539, 1990), and that has improved the practicality of the electro-luminescent device. Since then, numerous academic and industrial research studies based on electro-luminescent devices have been established.
Because an electro-luminescent device, which is able to luminesce under a low driving voltage, possesses a wide visual angle up to about 160.degree., is capable of displaying full color, and has a response short time, it possibly will become the next-generation flat panel color display. Since the development on the electro-luminescent device is nearly complete, applying the electro-luminescent device on applications including small-size display panels, outdoor display panels, and monitors is expected.
Recently, the prototype of a full-color organic electro-luminescent device utilizing small molecules has been made through development of materials. However, since the spin-coating process for coating the polymer solution layer cannot precisely define the locations of pixels of different colors, the development of an organic electro-luminescent device polymer is unrealized.
Of the principles of displaying full-color images on an organic electro-luminescent device, there are two main techniques are currently available: the direct-type full-color displaying technique, and the indirect-type full-color displaying technique. By using the direct-type full-color displaying technique, an OEL device displays colors through different pixels respectively, wherein each of the pixels is made to emit one of the pre-determined colors. On the other hand, an OEL device that applies the indirect-type full-color displaying technique colors images through a additional component such as a color filter or a color conversion layer located over the light-emitting layer.
There are various methods for fabricating an OEL device that applies the direct-type full-color displaying technique. Referring to FIG. 1, as described in ROC patent number 301,802, ROC patent number 318,284, and ROC patent number 318,966, micro-cavities of certain depths 110 are formed over a provided substrate 100 for displaying full-color images. The multi-thickness filler dielectric mirror 102 on the substrate 100 works as a quater-wave stack. The summation on the thickness of an indium tin oxide (ITO) layer 104 and a light-emitting layer 106 together with the local thickness of the multi-thickness filler dielectric mirror 102 constructs the depths of the micro-cavities. According to optical interference occurring within the micro-cavities, the electro-luminescent (EL) spectrum of an OEL device is changed through defining the depths of the micro-cavities. A metal layer 108 is formed on the foregoing structure and used as an electrode. The EL spectrum obtained from OEL devices of different structures is shown in FIG. 2. EL spectrum 200 is obtained from a non-cavity OEL device, and EL spectrum 202 is obtained from a multimode-cavity OEL device that contains peaks at several wavelengths. By precisely defining the depths of micro-cavities, an EL spectrum containing peaks at desired wavelengths, such as a red, green, and blue, can be obtained. However, the fabrication process of the foregoing structure requires very advanced technologies, thus the product is not cost competitive.
In ROC patent number 294,842, a stacking method is provided for fabricating a direct-type full-color OEL device. As shown in FIG. 3, a blue-light OEL device 302 and a red-light OEL device 304 are stacked on a substrate 300. Every pixel on the display panel formed by this method is capable of displaying either blue or red light, or both. By applying the foregoing method, a full-color display panel consisting of stacked red-light device, green-light device, and blue light OEL device can be fabricated. Since all OEL devices have to be precisely aligned by pixels, this increases the difficulty of the method. Furthermore, the electrode layers between the OEL devices degrade the intensities of the emitted light.
A method for fabricating a direct-type full-color OEL device that uses X-Y addressing pattern is shown in FIG. 4. As provided in U.S. Pat. Nos. 5,294,869 and 5,294,870, the method includes forming vertical shadow mask 404 on an ITO layer 402 over a substrate 400, and filling OEL materials 406 into the spaces partitioned by the vertical shadow mask 404. Then, an electrode layer 408 is formed by evaporation with a tilted angle on the filled OEL materials as a part of the driving circuit. The partitioned spaces containing the filled OEL materials 406 for emitting light of different wavelengths act as pixels. By filling proper OEL materials, a display panel consisting of such pixels is able to display full-color images under the operation of a driving circuit. Because the OEL materials are filled into the partitioned spaces by performing several deposition process, the fabricating cost is relatively high. In addition, since the OEL materials can only be filled into the partition spaces through deposition processes, the foregoing method is not suitable for polymer electro-luminescent device layers, which needs to be spin-coated onto the substrate.
In FIGS. 5A through 5E, developed by Professor Kido, a method for fabricating a direct-type full-color OEL device that applies photo-bleaching technique is illustrated. Referring to FIG. 5A, an ITO layer 502 is first formed on substrate 500. As shown in FIG. 5B, a dye-doped layer 504 for emitting red light is coated on the ITO layer 502. Then, referring to FIGS. 5C and 5D, by applying masks, 506a and 506b, and external irradiation, the conjugated structures of the energy gaps in desired portions of the dye-doped layer 504, 504a and 504b, are restructured, and then enlarged. The portions of the dye-doped layer 504, 504a and 504b, are able to emit green light and blue light by changing the energy gap. The electrode layer 508 is then formed on the pixels 504, 504a, and 504b. The electrode layer 508 and the ITO layer are connected to a driving circuit as shown in FIG. 5F. Since the energy gaps of the dye-doped material are directly related to the quality of displayed colors, the destruction of conjugation of exposing the dye-doped material under external irradiation will reduce electro-luminescent efficiency and increase the turn-on voltage. So, currently, this method is not practical.
A method provided by Yang Yang using an ink-jet printer, instead of a spin-coater, to coat polymer is able to reduce the waste of polymer in the fabrication process (Science, Volume 279, p.p. 1135, 1998). However, this new method still has some problems to be overcome, such as developing a better solution and resolving the clogging problem occurring at the ink-jet head.
Some methods have also developed for fabricating an indirect-type full-color OEL device. For example, a method developed by TDK Inc. that adds a color filter for displaying full-color images is shown in FIG. 6. Referring to FIG. 6, a red-green-blue color filter 612 is placed on the pixels of a conventional white-light OEL device. The provided conventional white-light OEL device consists of substrate 600, ITO layer 602, hole transport layer 604, white-light emitting layer 609, electron transport layer 608, and electrode layer 610. Since a color filter has to be attached on the white-light OEL device in order to display color images, the luminance of the color display device is inevitably suppressed.
Referring to FIG. 7, another method introduced by Idemitsu Kosan for fabricating an indirect-type full-color OEL device includes forming a color conversion layer 712 on a conventional blue-light OEL device. Similar to the previous method shown in FIG. 6, with the presence of a color conversion layer 712, the blue light emitted from a conventional blue-light OEL device is converted into a pixel set consisting of red, green, and blue lights. The conventional OEL device consists of a glass substrate 700, an ITO layer 702, a hole transport layer 704, a blue-light emitting layer 706, an electron transport layer 708, and an electrode layer 710. Even though the OEL device is capable of displaying color images with an additional color conversion layer 712, the presence of an additional color conversion layer 712 degrades the luminance of the device.